PTC heater assemblies are well known in the art. A PTC or positive temperature coefficient device is a semiconductor which has an electrical resistance which is temperature sensitive. The electrical resistance of the PTC device varies proportionately with temperature. PTC devices are available as ceramics or polymers and are well known for use in temperature sensors, current limiters and heaters. Their usefulness as a heater is particularly attractive due to the fact that a self regulating heater can be constructed. When a current is passed through a PTC device, it produces heat by virtue of the internal resistance of the PTC device and the resultant current is similar to that of other resistance heaters except that at a certain predetermined temperature (curie point or autostabilizing temperature), the resistance begins to increase virtually exponentially causing the power to decrease thereby autostabilizing the PTC device at a particular predetermined temperature. The temperature at which the PTC device autostabilizes will vary depending upon the specific PTC device. In the present invention, the autostabilizing temperature of the PTC device is useful because it can be established at a temperature which is below the ignition temperature of the heaters environment.
PTC heaters have not been particularly successful in the prior art when used in hostile environments such as in the chemical processing industry. In such hostile environments, strong oxidizers, free halogen ions and strong reducing acids contribute to the degregation of PTC assemblies.
In conventional resistance heaters utilized in hostile environments, the use of plastics and in particular fluoropolymers has proven successful but such plastics were unpractical with respect to their use in protecting PTC devices because a plastic with sufficient thickness to resist the hostile environment generally produces sufficient thermal resistance to increase the temperature of the PTC device above its autostabilizing temperature rendering it ineffective as a heating device.